1. Field of the Invention
The present invention relates to a voltage change detection device that detects whether or not a power supply voltage has changed to cross a predetermined detection potential.
2. Description of the Related Art
An apparatus for preventing a circuit from malfunctioning due to a reduction or increase of a power supply potential below or above a predetermined potential (throughout the specification, the term “potential” stands for electrical potential) is known in the art. See, for example, Japanese Patent Kokai No. H06-109781 (Patent Literature 1).
FIG. 1 is a circuit diagram of a conventional voltage change detection device 90. A drain of a DMOS 91 which is a depletion type NMOS field effect transistor is connected to a power supply potential VDD and a gate and a source thereof are connected to each other at a node S1. A source of an NMOS 92 which is an NMOS field effect transistor is connected to a ground potential VSS and a gate and a drain thereof are connected to each other at a node S2. The node S1 and the node S2 are connected to each other and the node S2 is connected to one input of a comparator 95. A drain and a gate of an NMOS 93 which is an NMOS field effect transistor are connected to the power supply potential VDD and a source thereof is connected to a node S3. A source and a gate of a DMOS 94 which is a depletion type NMOS field effect transistor are connected to the ground potential VSS and a drain thereof is connected to the node S3. The node S3 is connected to the other input of the comparator 95. The comparator 95 compares a reference potential REF1 provided to the one input thereof and a comparison potential REF2 provided to the other input thereof and changes the voltage level of an output OUT thereof from VDD to 0V when the comparison potential REF2 is higher than the reference potential REF1.
FIG. 2 illustrates a relationship between the reference potential, the comparison potential, and the detection potential in the conventional voltage change detection device 90. The vertical axis represents voltage and the horizontal axis represents the power supply potential VDD. The reference potential REF1 is a threshold voltage of the NMOS 92. Here, the threshold voltage is a voltage between the source and gate which allows conduction between the drain and source of the NMOS 92 or allows a constant current to flow therebetween. The term “threshold voltage” used in the following description has the same meaning as this threshold voltage. The threshold voltage of the NMOS 92 may vary due to manufacturing deviation or temperature changes. In FIG. 2, a reference potential REF1 of the NMOS 92 whose threshold voltage is high is shown as a reference potential R1H and a reference potential REF1 of the NMOS 92 whose threshold voltage is low is shown as a reference potential R1L. The reference potential R1H is higher than the reference potential R1L since the reference potential REF1 increases as the threshold voltage of the NMOS 92 increases. The DMOS 91 serves as a high resistor so that the reference potentials R1H and R1L undergo almost no change as the power supply potential VDD changes.
The comparison potential REF2 is lower than the power supply potential VDD by the threshold voltage of the NMOS 93. The threshold voltage of the NMOS 93 may also deviate due to manufacturing deviation or the like. In FIG. 2, a comparison potential REF2 of the NMOS 93 whose threshold voltage is high is shown as a comparison potential R2H and a comparison potential REF2 of the NMOS 93 whose threshold voltage is low is shown as a comparison potential R2L. The comparison potential REF2 decreases as the threshold voltage of the NMOS 93 increases since the comparison potential REF2 is lower than the power supply potential VDD by the threshold voltage of the NMOS 93. Conversely, the comparison potential REF2 increases as the threshold voltage of the NMOS 93 decreases. Therefore, the comparison potential R2H is lower than the comparison potential R2L. The DMOS 94 serves as a high resistor so that the comparison potentials R2H and R2L increase as the power supply potential VDD increases and decrease as the power supply potential VDD decreases.
The threshold voltage of the NMOS 92 and the threshold voltage of the NMOS 93 change in the same direction due to manufacturing deviation or the like. When the threshold voltage deviates upward, the reference potential REF1 increases while the comparison potential REF2 decreases. When the threshold voltage deviates downward, the reference potential REF1 decreases while the comparison potential REF2 increases. In this manner, the reference potential REF1 and the comparison potential REF2 change in opposite directions. Thus, there is a great difference between a detection potential VH1 of the comparator 95 when the threshold voltage is high and a detection potential VH2 thereof when the threshold voltage is low.
The following problems occur when the deviation of the detection potential becomes large. For example, when the detection potential is deviated upward, the voltage change detection device changes the voltage level of the output OUT to a level higher than a desired detection potential, thereby causing a problem in that the operation of a voltage detection target circuit (not shown) is stopped even when battery power remains and is still able to supply a sufficient power supply potential. On the other hand, when the detection potential is deviated downward, the voltage change detection device changes the voltage level of the output OUT to a level lower than the desired detection potential, thereby causing a problem in that a voltage lower than the operation-guaranteed voltage is provided to the circuit and thus the circuit does not operate normally.
FIG. 3 illustrates a relationship between temperature and a detection potential of the voltage change detection device 90. The vertical axis represents detection potential and the horizontal axis represents temperature. In FIG. 3, the threshold voltage of the NMOS is denoted by a symbol “TT” when it is standard, “SS” when it is high, and “FF” when it is low and the threshold voltage of the DMOS is denoted by a symbol “DS” when it is high and “DF” when it is low. An upper limit value of a desired detection potential is denoted by “upper limit” and a lower limit value thereof is denoted by “lower limit”. For example, the upper limit value is an upper limit of the operating voltage of a voltage detection target IC circuit and the lower limit value is a lower limit thereof. The upper limit value is 1.3V and the lower limit value is 1.0V.
Since the comparator 95 changes the voltage level of the output OUT when the comparison potential REF2 is higher than the reference potential REF, the detection potential is obtained as follows. When the drain-source voltage of the NMOS 92 is denoted by “Vt1” and the drain-source voltage of the NMOS 93 is denoted by “Vt2”, VDD−Vt2>Vt1 since comparison potential REF2>reference potential REF1. This can be rewritten as VDD>Vt1+Vt2. That is, the comparator 95 changes the voltage level of the output OUT when VDD is higher than Vt1+Vt2. Thus, the value of Vt1+Vt2 is the detection potential, and the comparator 95 detects whether or not the power supply potential VDD has changed to cross this detection potential. Since the detection potential is the sum of the drain-source voltage of the NMOS 92 and the drain-source voltage of the NMOS 93, deviation of the detection potential increases when the threshold voltages of the NMOS 92 and the NMOS 93 have deviated due to temperature and/or manufacturing conditions. As shown in FIG. 3, the detection potential is higher than the upper limit when the threshold voltage is high as denoted by a symbol “SS” depending on temperature and is lower than the lower limit when the threshold voltage is low as denoted by a symbol “FF”. Thus, the detection potential may not be detected within the desired detection potential due to deviation in the threshold voltage.
In the case where the conventional voltage change detection device is used, deviation in the detection potential is increased when the threshold voltage of a field effect transistor is deviated due to manufacturing deviation or the like and the detection potential cannot be detected within a desired detection potential, thereby causing malfunction or the like of a voltage detection target IC circuit.